As technology for recording and reproducing digital data, there is a data recording technology using optical disks (including magneto-optical disks) such for example as CD (Compact Disk), MD (Mini-Disk), and DVD (Digital Versatile Disk) for recording media. The optical disk is a generic term for recording media that are disks having a metallic thin plate protected with plastic and irradiated with laser light to read a signal through change in the reflected light.
Optical disks for example include reproduction-only types known as CD, CD-ROM, DVD-ROM and the like and user data recordable types known as MD, CD-R, CD-RW, DVD-R, DVD-RW, DVD+RW, DVD-RAM and the like. The recordable types allow data to be recorded thereon by using a magneto-optic recording method, a phase change recording method, a dye film change recording method and the like. The dye film change recording method is also referred to as a write-once recording method, which allows data recording only once and does not allow rewriting. The dye film change recording method is therefore suitable for data storing purposes and the like.
On the other hand, the magneto-optic recording method and the phase change recording method allow data rewriting, and are used for various purposes including recording of various contents data such as music, video, games, application programs and the like.
To record data on a recordable disk by the magneto-optic recording method, the dye film change recording method, the phase change recording method or the like requires guiding means for tracking a data track. Thus, a groove is formed in advance as a pregroove, and the groove or a land (a portion of trapezoidal cross section sandwiched between grooves) is used as a data track.
It is also necessary to record address information so that data can be recorded at predetermined positions on the data track. The address information may be recorded by wobbling the groove.
Specifically, the track for recording data is formed in advance as a pregroove, for example, and side walls of the pregroove are wobbled in correspondence with the address information.
This makes it possible to read addresses from wobbling information obtained as reflected light information at the time of recording and reproduction and therefore record and reproduce data at desired positions even when for example pit data or the like indicating addresses is not formed in advance on the track.
Thus adding the address information as a wobbling groove eliminates the need for providing for example discrete address areas on the track and recording addresses as pit data, for example. Since the address areas are not required, real data recording capacity can be correspondingly increased. Incidentally, absolute time (address) information represented by such a wobbled groove is referred to as ATIP (Absolute Time In Pregroove) or ADIP (Address In Pregroove).
Recently, it has become necessary to record various information on the disk in advance, as with the address information, in addition to the address information and information recorded and reproduced by the user.
Specifically, as prerecorded information recorded on the disk in advance, disk information indicating conditions for recording on the disk, for example a recording linear velocity, a recommended value of laser power and the like, and copy protect information for excluding hacked apparatus and the like are desired to be recorded. The copy protect information is particularly important.
A known method for prerecording various information on the disk is to form embossed pits on the disk.
Considering high-density recording and reproduction on an optical disk, however, the method of prerecording by embossed pits is disadvantageous.
High-density recording and reproduction on an optical disk requires a reduction in groove depth. In the case of a disk having a groove and embossed pits produced simultaneously by a stamper, it is very difficult to make depth of the groove and depth of the embossed pits different from each other. Thus, the depth of the embossed pits has to be the same as the depth of the groove.
However, when the depth of the embossed pits is reduced, a signal of good quality cannot be obtained from the embossed pits.
For example, a volume of 23 GB (gigabytes) can be recorded and reproduced on an optical disk 12 cm in diameter by recording and reproducing phase change marks at a track pitch of 0.32 μm and a linear density of 0.12 μm/bit on a disk having a cover (substrate) thickness of 0.1 mm, using a laser diode having a wavelength of 405 nm and an objective lens having an NA=0.85 as an optical system.
In this case, the phase change marks are recorded and reproduced on a groove formed in a spiral fashion on the disk. In order to suppress media noise for higher density, a groove depth of about 20 nm, that is, 1/13 to 1/12 of a wavelength λ is desirable.
In order to obtain a signal of good quality from embossed pits, on the other hand, a depth of the embossed pits is desired to be λ/8 to λ/4. After all, a good solution as a common depth of the groove and the embossed pits has not been obtained.
Because of such a situation, a method of prerecording information by other than embossed pits is desired.